Add optional clientId/clientSecret config for a Mixcloud application. When both are defined the source runs the OAuth authorization flow (redirecting through /api/mixcloud/callback), persists the received access token next to the config like the spotify source, and reads listens from /me/listens/ -- which includes the private and pro listening history the public endpoint does not expose. The username is discovered from the authorized user's profile. With only a username the source behaves exactly as before, so existing configs keep working unchanged.